1. Field of the Invention
This invention relates to the field of amplifiers, and particularly to CMOS amplifiers having high gain.
2. Description of the Related Art
A wide variety of circuit designs have been used to realize integrated circuit (IC) amplifiers. Operational amplifiers are employed extensively, and usually comprise a differential input stage and a single-ended output stage.
Although such amplifiers have served many practical purposes, their performance is not fully satisfactory in certain respects. For example, the gain of many prior amplifiers tends to be low, or to vary to an undesired degree with changes in load, due to variations in ambient temperature, for example. When used in an application such as a high precision voltage reference, such variation can adversely affect circuit performance to an unacceptable degree.
One high gain IC amplifier is described in U.S. Pat. No. 4,857,862 to Brokaw. The operating point of this amplifier is automatically adjusted to keep the amplifier balanced over changes in load. However, this amplifier is designed to operate only with bipolar transistors; for example, it is arranged so that the effects of several base currents compensate one other, thereby improving performance. As a result, this bipolar amplifier is ill-suited for use in a CMOS circuit.